1. Field of the invention
The present invention relates to a control device for controlling the operation of a travel motor which is mounted to a vehicle.
2. Description of the Related Art
For example, vehicles such as electric motor vehicles and hybrid vehicles are equipped with a travel motor and a control device to control the operation of the travel motor. The travel motor works as a travel power source for the vehicle. The control device controls a current vector of a current which is supplied to the travel motor based on the rotation angle of the rotor of the travel motor. For example, Japanese patent laid open publication No. H10-215504 discloses such a conventional technique to control the current vector of the current which is supplied to the travel motor.
It is therefore possible for the control device capable of controlling the operation of the travel motor to detect the rotation angle of the travel motor with high accuracy. In general, a rotation angle sensor is used to detect the rotation angle of the rotor of a travel motor. For example, a resolver is used as the rotation angle sensor, and there is a resolver digital converter (RDC) of a tracking type which is a well known device to convert an analogue signal output from the resolver to a digital signal. Japanese patent No. JP 3442316 has disclosed such a resolver digital converter.
In general, the resolver is comprised of a primary coil and secondary coils. The resolver outputs a rotation detection signal when receiving an exciting signal F(t) having a constant frequency by the primary coil. In more detail, when the exciting signal F(t) is supplied to the primary coil of the resolver, the secondary coils output a first rotation detection signal “F(t)·sin θ” and a second rotation detection signal “F(t)·cos θ”, where the first rotation detection signal “F(t)·sin θ” is obtained by performing the amplitude modulation of the exciting signal F(t) with sin θ, and the second rotation detection signal “F(t)·cos θ” is obtained by performing the amplitude modulation of the exciting signal F(t) with cos θ.
The variable θ is called to as the “resolver electrical angle θ or electrical angle θ” which is obtained by multiplying a mechanical angle as the rotation angle of a motor “n” times, where “n” is an integer of not less than 1. The value “n” is the number of rotations of the electrical angle θ per one rotation of the motor (that is, a multiple rate of the electrical angle θ to the mechanical angle). The value “n” is in general called to as a “shaft multiple angle”.
The RDC (resolver digital converter) outputs the rotation angle output value φ in digital form as the detection value of the electrical angle θ. The RDC performs a tracking loop calculation to feed back the rotation angle output value φ as the input of the RDC every constant time period.
In the tracking loop calculation, the RDC outputs a next rotation angle output value φ in order to satisfy the relationship of φ=θ by the following steps (a-1), (a-2), (a-3), and (a-4):
(a-1) obtaining the first output signal F(t)·sin(θ−φ)=[F(t)·sin θ·cos φ]−[F(t)·cos θ·sin φ] by multiplying the first rotation detection signal F(t)·sin θ and the second rotation detection signal F(t)·cos θ transferred from the resolver and sin φ and cos φ which are obtained based on the output current angle value φ of the RDC;
(a-2) obtaining the second output signal of sin(θ−φ) as an error deviation ε by performing the synchronous detection of the first output signal F(t)·sin(θ−φ) and removing the component of the exciting signal F(t) from the value after the synchronous detection;
(a-3) integrating the value obtained by multiplying the error deviation ε by a predetermined gain; and
(a-4) further integrating the integrated value obtained by (a-3) in order to obtain the next rotation angle output value φ.
Japanese patent publication No. JP 3442316 does not show the process to multiply the error deviation ε by a gain. However, in order to optimize the tracking loop process by using the error deviation ε of a small value, it is necessary to multiply the error deviation ε by a constant gain value.
The control device for a travel motor detects the rotation angle of the travel motor based on the rotation angle output value φ from the RDC, and determines a current vector of the current to be supplied to the travel motor based on the obtained rotation angle of the travel motor.
By the way, the RDC can perform an ideal operation and detect the rotation angle with high accuracy when the rotation detection signals in two phase transferred from the resolver have the same amplitude and there is no offset in those rotation detection signals.
Japanese patent publication No. JP 3365063 discloses the technique to correct the amplitude and offset of a rotation detection signal transferred from a resolver.
In the conventional technique disclosed in Japanese patent publication No. JP 3365063, the rotation angle of a rotary body is detected based on sine-wave signals in A phase and B phase which are different by 90° and output from a sine-wave encoder. This conventional technique performs the following operations every output period of the sine-wave encoder:
(b-1) the maximum value and the minimum value of the sine-wave signals in A phase and B phase are obtained;
(b-2) an amplitude change ratio is calculated based on a ratio between the amplitude of each of the sine-wave signals in A phase and B phase obtained based on the maximum value and the minimum value and a reference amplitude; and
(b-3) the signal values in A phase and B phase are corrected based on the amplitude change ratio calculated by using the maximum value and the minimum value, where those calculated signal values in A phase and B phase are used to detect the rotation angle of the rotary body such as a travel motor.
In addition, the conventional technique disclosed in Japanese patent publication No. JP 3365063 calculates the offset value of each of the signal values in A phase and B phase by dividing a difference between the maximum value and the minimum value by 2, and corrects offset values in A phase and B phase by subtracting the signal values in A phase and B phase to be used to detect the rotation angle of the rotary body from the previous offset value based on the maximum value and the minimum value previously obtained one period before.
Applying the conventional technique disclosed in JP 3365063 to the RDC allows the amplitude and offset of each of a sin θ component and a cos θ component of the rotation detection signals transferred from the resolver to be corrected.
However, the resolver has another problem to have a manufacturing error in shape of the rotor and the stator thereof. There is a possibility for the resolver not to output rotation detection signals of a complete sin θ waveform and a complete cos θ waveform based on the fluctuation in phase of the rotor and the stator. This will be referred to as the “output waveform distortion of a resolver”.
When the resolver outputs the rotation detection signals having a waveform distortion, the RDC does not generate a correct output, and this increases an error component in the rotation angle output value φ output from the RDC (in other words, the detection accuracy of the RDC to detect the rotation angle of the travel motor is decreased), and this decreases the control accuracy of the rotary body such as the travel motor. The conventional technique disclosed in JP 3365063 does not suppress the deterioration of the rotation detection accuracy caused by the distortion of the output waveform of the resolver.
FIG. 9 is an explanatory view showing a problem caused by a conventional technique when a resolver outputs a detection signal with a waveform distortion.
For example, as designated by the solid line in FIG. 9, the resolver outputs the rotation detection signal having a distortion of an output waveform, the RDC outputs a non-linear rotation angle value φ which is not changed in proportion to the elapse of time even when the travel motor is rotates at a constant speed. Because the control device for a travel motor obtains the rotation angle of the drive motor based on the rotation angle output value φ transferred from the RDC and determined the current vector to be supplied to the travel motor, it would be difficult for the control device to output the optimum current vector. This decreases the accuracy to control the rotation angle of the travel motor. In particular, the long and dash line in FIG. 9 shows the ideal rotary angle detection value φ when the travel motor rotates at a constant speed.
Further specifically, when the control device for a travel motor instructs the travel motor to rotate at a constant rotation speed in order to drive the vehicle at a constant speed according to the information instructed by the driver of the vehicle such as by the stroke of the accelerator pedal, the control device controls the travel motor to operate based on a different rotation angle which is different from an actual rotation angle of the travel motor by the rotation angle detection error caused by an output waveform distortion from the resolver. This increases or decreases the output torque of the travel motor, and thereby causes undesired acceleration or deceleration of the vehicle. This gives uncomfortable driving to the passengers and the driver of the vehicle. Through the specification, the word “acceleration” includes both a positive acceleration and a negative acceleration (that is, deceleration) unless otherwise indicated.